1. Field of the Invention
This invention relates to an electron microscope charge-up prevention method and an electron microscope capable of eliminating electricity of a specimen charged-up by a specimen charge-up phenomenon occurring in a scanning electron microscope, a transmission electron microscope, etc.
2. Description of the Related Art
Nowadays, an electron microscope using an electron lens as well as an optical microscope using an optical lens and a digital microscope is used as an enlargement observation apparatus for enlarging a microbody. The electron microscope is provided by electronically optically designing an image formation system such as an optical microscope as the travel direction of electrons is refracted freely. The available electron microscopes include a transmission electron microscope, a reflection electron microscope, a scanning electron microscope, a surface emission electron microscope (field-ion microscope), and the like. The transmission electron microscope uses an electron lens to form an image of electrons passing through a specimen, a sample, etc. The reflection electron microscope forms an image of electrons reflected on the surface of a specimen. The scanning electron microscope scans a convergent electron beam over the surface of a specimen and uses secondary electrons from the scanning points to form an image. The surface emission electron microscope (field-ion microscope) forms an image of electrons emitted from a specimen by heating or ion application.
The scanning electron microscope (SEM) is an apparatus for using a secondary electron detector, a reflection electron detector, etc., to take out secondary electrons, reflection electrons, etc., occurring upon application of a thin electron beam (electron probe) to an objective specimen and displaying an image on a display screen of a CRT, LCD, etc., for the operator mainly to observe the surface form of the specimen. On the other hand, the transmission electron microscope (TEM) is an apparatus for allowing an electron beam to pass through a thin-film specimen and providing electrons scattered and diffracted by atoms in the specimen at the time as an electron diffraction pattern or a transmission electron-microscopic image, thereby enabling the operator mainly to observe the internal structure of a substance.
When an electron beam is applied to a solid specimen, it passes through the solid by energy of the electrons. At the time, an elastic collision, elastic scattering, and inelastic scattering involving an energy loss are caused by the interaction between the nucleuses and the electrons making up the specimen. As inelastic scattering occurs, the intra-shell electrons of the specimen elements and X-rays, etc., are excited, and secondary electrons are emitted, the energy corresponding thereto is lost. The emission amount of the secondary electrons varies depending on the collision angle. On the other hand, reflection electrons scattered backward by elastic scattering and emitted again from the specimen are emitted in the amount peculiar to the atom number. The scanning electron microscope uses the secondary electrons and the reflection electrons. The scanning electron microscope applies electrons to a specimen and detects the emitted secondary electrons and reflection electrons for forming an observation image.
Various factors in interfering with SEM observation exist; a representative one of factors caused by a specimen is a charge-up phenomenon occurring when a nonconductive specimen is observed. The charge-up is a phenomenon in which the application plane is charged positively or negatively because of the difference between the charges that incident charged particles and emitted charged particles have. When charge-up occurs, emitted secondary electrons are accelerated or brought back and it is made impossible to provide a good image formation characteristic; rarely, no images are formed.
When an electron beam having negative charges is incident on a bulk-like specimen, if the specimen is conductive, the charges travel through the specimen and are grounded; if the specimen is nonconductive, the charges of the incident electrons cannot escape from the surface of the specimen and the specimen itself is charged up. As a charge-up phenomenon, abnormal contrast occurs in the observation visual field or contrast appears like a belt and it becomes difficult to observe a secondary electron image. As charge-up becomes heavy, a drift of the observation visual field (the visual field slowly moves or suddenly moves rapidly) also occurs.
Thus, if the observation sample charged negatively by application of an electron beam is observed, various image faults occur. Various techniques for preventing the detrimental effects of charge-up are developed. For example, a method of applying a metal coating of gold, etc., to the surface of a nonconductive specimen is known. However, this method has disadvantages in that the metal coating is cumbersome and the specimen after observation is not restored to the former state.
Available as a technique of eliminating charges of a charged-up specimen is a method of applying an electron beam of low acceleration voltage with the generation efficiency from the specimen exceeding 1 (JP-A-7-14537). In this method, however, if the charge energy of the specimen exceeds the energy of the electron beam applied for eliminating charge, the applied electron beam is repelled and cannot arrive at the specimen and charge cannot be eliminated; this is a problem. The method also involves a problem of difficult setting of acceleration voltage. Although the acceleration voltage must be set to such acceleration voltage at which the generation efficiency of secondary electrons becomes 1, the acceleration voltage at which the generation efficiency becomes 1 varies depending on the material and shape of the specimen and thus such acceleration voltage at which the generation efficiency becomes 1 must be found out while the acceleration voltage is adjusted. If the specimen is charged up, the initial velocity of a primary electron and the beam arrival speed at the incidence time on the specimen differ and thus it becomes difficult to set the optimum acceleration voltage. Even if the acceleration voltage at which the generation efficiency becomes 1 is found out, if electrons of high acceleration voltage are applied to the specimen in the observation process conducted so far, the specimen is already charged negatively and thus an image fault caused by charge-up occurs. If applying electrons of acceleration voltage at which the generation efficiency becomes 1 is continued for a long time (for example, about several hours), charge may be able to be eliminated, but the method is not realistic.
A method of erasing charges accumulated on the surface of a specimen by applying an electron beam of acceleration voltage for generating electrons of the opposite polarity is also available (JP-A-7-14537). However, this method is a method of observing before accumulating charges is started, namely, a measure to prevent charging and is not an aggressive charge elimination method of eliminating charges.
Developed as another technique is a technique of sensing whether or not a specimen is charged up and taking measures against charging if the specimen is determined to be charged up. However, the following problems are involved in sensing charge-up: A dedicated facility to sensing charge-up becomes necessary and various phenomena of charge-up occur and thus it is difficult to correctly sense charge-up, etc. Further, the method is to prevent charge-up and is not a measure to be taken when charge-up occurs.
A method of placing the inside of a specimen chamber in atmospheric pressure and exposing a specimen to air for eliminating electricity is also available. In this method, however, electricity may be insufficiently eliminated and evacuation needs to be again performed from the beginning to again conduct observation; the method also has disadvantage in that it requires time and labor.
Further, a method of observing in a low vacuum, a method of using an electronic shower generator to apply an electronic shower, a method of using an ion shower generator to apply an ion shower (JP-A-10-12684), or a method of providing a control electrode (JP-A-5-343021) is also available. However, the methods require additional dedicated facilities and cannot easily be executed.
Thus, every method has the disadvantages and an apparatus capable of easily eliminating charge of a specimen is expected. It is therefore an object of the invention to provide an electron microscope charge-up prevention method and an electron microscope capable of simply eliminating electricity of a charged-up specimen without providing any additional dedicated facility.
In order to accomplish the object above, the following means are adopted. According to the present invention, there is provided a method for preventing an electron microscope from being charged up, the method comprising;
setting the maximum value of acceleration voltages of primary electrons applied to a specimen in the past as a start acceleration voltage for electricity elimination;
applying an electron beam to the specimen with the acceleration voltage set to the start acceleration voltage; and
gradually dropping the acceleration voltage from the start acceleration voltage to a termination acceleration voltage for electricity elimination, the termination acceleration voltage being an acceleration voltage value with which a landing acceleration voltage of the electron beam on a specimen face is placed in a range where a secondary electron emission efficiency of the specimen becomes 1 or more.
The above-mentioned electron microscope charge-up prevention method may further comprises: setting the termination acceleration voltage to an acceleration voltage value with which the landing acceleration voltage is the maximum value at which the secondary electron emission efficiency of the specimen becomes 1.
Further, the above-mentioned electron microscope charge-up prevention method may further comprises: setting the termination acceleration voltage to an acceleration voltage value with which the landing acceleration voltage is placed in an area wherein the secondary electron emission efficiency of the specimen exceeds 1.
In the above-mentioned electron microscope charge-up prevention method, while the acceleration voltage is dropped, the acceleration voltage may be applied continuously or discretely until the specimen charged negatively becomes uncharged or is charged positively.
The above-mentioned electron microscope charge-up prevention method may further comprises: comparing a plurality of specimens with respect to the maximum landing acceleration voltage; and setting the termination acceleration voltage to the lowest value of the maximum landing acceleration voltages or less.
Further, the above-mentioned electron microscope charge-up prevention method may further comprises: executing at least one simple observation image acquiring function, the simple observation image acquiring function automatically setting a plurality of simple image observation conditions with the acceleration voltage changed, acquiring a simple observation image for each acceleration voltage, and listing the simple observation images on a display; determining the maximum acceleration voltage at which the specimen is not charged up on the basis of simple observation images acquired; and setting the termination acceleration voltage to the maximum acceleration voltage at which the specimen is not charged up.
In the above-mentioned electron microscope charge-up prevention method, an area wherein the electron beam may be applied to the specimen when the charge elimination is executed may be set wider than an area when an observation is executed.
Further, in the above-mentioned electron microscope charge-up prevention method, an area wherein the electron beam may be applied to the specimen when the charge elimination is executed may be set to an area wherein a charge-up occurs or a slightly larger area than the area wherein the charge-up occurs.
In order to achieve the object of the present invention, there is also provided an electron microscope comprising:
an electron gun for applying an electron beam based on an acceleration voltage to a specimen;
an acceleration voltage application section for applying the acceleration voltage to the electron gun;
an acceleration voltage adjustment section for adjusting the acceleration voltage applied to the electron gun;
a first voltage setting section for recording the maximum value of the acceleration voltages of primary electrons applied to the specimen and setting the maximum acceleration voltage as a start acceleration voltage for an charge elimination; and
a second voltage setting section for setting an acceleration voltage value with which a landing acceleration voltage of the electron beam on a specimen face is placed in a range where a secondary electron emission efficiency of the specimen becomes 1 or more as a termination acceleration voltage for the charge elimination,
wherein when the electricity elimination is executed, the acceleration voltage adjustment section gradually drops the acceleration voltage of the electron beam applied to the specimen from the start acceleration voltage to the termination acceleration voltage.
In the above-mentioned electron microscope, the termination acceleration voltage may be set to an acceleration voltage value with which the beam arrival acceleration voltage is the maximum value at which the secondary electron emission efficiency of the specimen becomes 1.
Further, in the above-mentioned electron microscope, the termination acceleration voltage may be set to an acceleration voltage value with which the landing acceleration voltage is placed in an area wherein the secondary electron emission efficiency of the specimen exceeds 1.
In the invention, the electron gun for observation is used to eliminate electricity of the charged-up specimen, so that additional special apparatus of charge-up sensing and electricity elimination facilities, etc., need not be provided and the cost can be reduced. The acceleration voltage for applying primary electrons to a specimen is lowered gradually from the acceleration voltage at which the primary electrons are not repelled by the electrons charged on the specimen to the acceleration voltage for removing the electrons charged on the specimen, whereby charge is eliminated. Thus, electricity of even a specimen in such a strong charge state repelling an electron beam in the method in the related art can be eliminated, and electricity elimination can be conducted effectively for various charge-up phenomena. The need for releasing or changing the vacuum state as in the method in the related art is also eliminated and electricity elimination can also be executed easily in a short time with the vacuum state maintained.